Three of the seven exoplanets—so called because they reside outside our solar system—reside within the habitable zone, where they are at the correct distance from their star to support liquid water, the key to life on Earth. In the right atmospheric conditions, all seven planets in the TRAPPIST-1 system, as it has been named, could theoretically hold liquid water.

"The TRAPPIST-1 system provides one of the best opportunities in the next decade to study the atmospheres around Earth-size planets," said Nikole Lewis, co-leader of the Hubble study, in a press release. Lewis is also an astronomer at the Space Telescope Science Institute in Baltimore.

The preliminary results are looking good for Earth-like planets. The planets are dense enough to be rocky like Earth and Mars, as opposed to gaseous like Jupiter and Saturn. The Hubble telescope also did not find the cloudy, hydrogen-rich atmospheres of gaseous planets on the two closest to the star, and future observations will search for the same clouds on the other planets.

Rocky planets with liquid water are extremely promising for the possibility of life. The Spitzer Space Telescope will resume its mission in the fall, while the Kepler Space Telescope is already refining the mass and density measurements and looking for even more planets in the TRAPPIST-1 system.

The big gun, however, will not come until 2018, when the James Webb Space Telescope goes into orbit. The James Webb telescope has instruments capable of detecting oxygen, ozone, methane and water: all signatures of Earth’s atmosphere. Detecting an atmosphere that even remotely resembles Earth would be a legitimate game-changer in the search for habitable planets.

There is still a problem, however, with the search for life. Even if one of the TRAPPIST-1 planets has an atmosphere exactly like Earth’s with plenty of liquid water on its surface, the system is 235 trillion miles or 40 light-years away. Even sending a message back and forth to TRAPPIST-1 would take 80 years, and with today’s space travel technology, taking a trip to TRAPPIST-1 is impossible.

Whoever does get to make that trip, however, is in for a sight unlike anything we could imagine on Earth. Despite having so many possibly Earth-like planets, TRAPPIST-1 is remarkably different from our solar system. First, the TRAPPIST-1 star is barely even a star. It is an object known as an ultra-cool dwarf, which is much smaller than our sun—a yellow dwarf—and burns at less than half the temperature.

You might think that the small ultra-cool dwarf would appear much smaller in the sky of the TRAPPIST-1 planets, but that is not the case, because the planets are much closer to the star than we are to the sun. Because the ultra-cool dwarf emits so much less energy, and because it is so much smaller than the sun, planets orbit at much closer distances to the star and still hold liquid water. All of the TRAPPIST-1 planets are closer to their star than Mercury is to our sun, so they see a sun very much like we do on Earth, albeit with a slightly different spectrum of radiation.

The most striking feature, however, would be seeing the other planets in the sky. Since all seven TRAPPIST-1 planets orbit so close to their star, they are all very close to each other. They are so close, NASA scientists say, that from the surface of some, you could see the geographical features and clouds of another planet with the naked eye. In some cases, they would look larger than the moon does in our sky. The most we can see of the other planets in our solar system is a dot that looks like an extra-bright star.

There are some other aspects of would-be day-to-day life on the TRAPPIST-1 planets that might throw someone used to living on Earth. NASA scientists think the planets may be tidally locked, meaning they spin at the same frequency that they revolve around their star. The end result is that one side of each planet is always in day while the other side is always in night. Apart from the shock of everlasting day or night, a planet like that could have weather unlike anything we have on Earth, as the daily rise and set of the sun have an enormous effect on the weather.

Having the star so close also could present a radiation problem for TRAPPIST-1 colonists. On Earth, we are far enough away from the sun that most of the hard-core ionizing radiation doesn’t reach us in a dangerous quantity. Being so much closer to their star, the TRAPPIST-1 planets do not necessarily have that same protection. There are, however, atmospheric conditions that could protect these planets.

Even though we may not be able to reach TRAPPIST-1 now, the star and its planets will be around for a long time. Our sun, which burns bright and hot is set to go out in a quick 5 billion years, but TRAPPIST-1 will burn for an astronomical several trillion years. The search for Earth-like planets is also about finding places that can support human life. When we have space travel figured out in a million years, TRAPPIST-1 might be where we go on vacation.

For now, the fact that this one system has seven Earth-like planets holds promise for millions more stars to hold a planet with that perfect combination of features to support Earth-like life.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.